Why does alkyne and alkene undergo addition reaction whereas alkane does not?

1 Answer
Apr 8, 2016

Alkanes are already bonded strongly, while alkenes and alkynes have weak pi-bonds. They want to undergo addition reactions to turn pi-bonds to stronger sigma's and become more stable.

Explanation:

Alkenes and alkynes are unsaturated - they have pi-bonds, so don't have the full number of hydrogen that they could have.

This means that they are more unstable than alkanes, since pi-bonds aren't as strong as sigma-bonds. The alkenes and alkynes want to form more sigma-bonds and have a structure more like an alkane, so they undergo addition reactions.

Addition reactions are where more atoms are added to the molecule, not swapped or taken away. This means that the pi-bonds have to be taken away and used as sigma-bonds with the new atoms, rather than the sigma-bonds already there being reattached - it's easier to break pi than sigma.

Alkanes do not undergo this reaction because they already only have single sigma-bonds, and so they cannot become more stable or stronger structurally - they are already at the peak, and so can only swap things around in substitution reactions.